Real time complete Stokes polarimetric imager based on a linear polarizer array camera for tissue polarimetric imaging

Passive Polarized Vision for Autonomous Vehicles: A Review

This review article aims to address common research questions in passive polarized vision for robotics. What kind of polarization sensing can we embed into robots? Can we find our geolocation and true north heading by detecting light scattering from the sky as animals do? How should polarization images be related to the physical properties of reflecting surfaces in the context of scene understanding? This review article is divided into three main sections to address these questions, as well as to assist roboticists in identifying future directions in passive polarized vision for robotics. After an introduction, three key interconnected areas will be covered in the following sections: embedded polarization imaging; polarized vision for robotics navigation; and polarized vision for scene understanding. We will then discuss how polarized vision, a type of vision commonly used in the animal kingdom, should be implemented in robotics; this type of vision has not yet been exploited in robotics service. Passive polarized vision could be a supplemental perceptive modality of localization techniques to complement and reinforce more conventional ones.

Fast and high-accuracy collinear reflection Mueller imaging polarimeter implemented with the compound calibration method

The collinear reflection Mueller matrix imaging polarimeter is suitable for characterizing thick samples with high-scattering depolarization such as biological tissues or in-situ living organs. Achieving fast detection and high measurement accuracy is vital to prevent artifacts and accurately assess polarization characteristics in these applications. This paper demonstrates a fast collinear reflection imaging polarimeter based on liquid crystal variable retarders (LCVRs-CRMMIP). We propose a novel compound calibration method (CCM), to the best of our knowledge, which enhances measurement accuracy through light intensity correction and an improved equivalent calibration sample model. This method surpasses the double-pass eigenvalue calibration method (dp-ECM), enhancing accuracy by over 23 times. Performance evaluations with standard samples, including mirrors, linear polarizers, and wave plates, reveal that the LCVRs-CRMMIP achieves rapid measurements (about 3 s) and high accuracy with an error of less than 0.0017.

Vortex retarder-based Stokes polarimeters: optimal data processing and autocalibration capability

We present a full Stokes polarimeter that utilizes a vortex retarder (VR) in conjunction with a polarization camera. We demonstrate its capability to estimate the full Stokes vector in a single shot with optimal precision and to autocalibrate the VR retardance, ensuring precise measurements even in dynamic environments where retardance is variable.

Low-Dimensional-Materials-Based Photodetectors for Next-Generation Polarized Detection and Imaging

The vector characteristics of light and the vectorial transformations during its transmission lay a foundation for polarized photodetection of objects, which broadens the applications of related detectors in complex environments. With the breakthrough of low-dimensional materials (LDMs) in optics and electronics over the past few years, the combination of these novel LDMs and traditional working modes is expected to bring new development opportunities in this field. Here, the state-of-the-art progress of LDMs, as polarization-sensitive components in polarized photodetection and even the imaging, is the main focus, with emphasis on the relationship between traditional working principle of polarized photodetectors (PPs) and photoresponse mechanisms of LDMs. Particularly, from the view of constitutive equations, the existing works are reorganized, reclassified, and reviewed. Perspectives on the opportunities and challenges are also discussed. It is hoped that this work can provide a more general overview in the use of LDMs in this field, sorting out the way of related devices for "more than Moore" or even the "beyond Moore" research.

Polarization angle information enhancement method based on polarimetric array imaging

Polarization imaging, based on the measurement of polarization parameters containing specific physical information, has found extensive applications across various domains. Among these parameters, polarization angle information plays a crucial role in revealing texture details. However, in practical scenarios, noise during image acquisition can lead to significant degradation of polarization angle information. To address this issue, we introduce a novel, to the best of our knowledge, polarization angle information enhancement method based on polarimetric array imaging. Our proposed method utilizes the principles of polarimetric array imaging to effectively restore texture information embedded within polarization angle images. Through the deployment of a self-designed polarimetric array imaging system, we conducted experiments in diverse scenes to validate the efficacy of our approach. The acquired polarization angle data were subjected to our method for enhancement. The experimental outcomes distinctly illustrate the noise suppression capabilities of our method, showcasing its ability to faithfully reconstruct intricate details obscured by substantial noise interference.

High-accuracy reconstruction of Stokes vectors via spatially modulated polarimetry using deep learning at low light field

Polarization measurement is generally performed in scenes with a low signal-to-noise ratio (SNR) such as remote sensing and biological tissue detection. The spatially modulated polarimeter can satisfy the real-time measurement requirements in low SNR scenes by establishing the mapping between photon spatial distribution and polarization information. However, accurately measuring the polarization state under low-light illumination becomes highly challenging owing to the interference of background noise. In this paper, a deep learning method is proposed and applied to the high-accuracy reconstruction of polarization information at low light field. A reinforced two-layer deep convolutional neural network is designed to respectively extract global and local features of noise in this method. Accurate photon spatial distribution can be obtained by fusing and processing these features. Experimental results illustrate the excellent accuracy achieved by the proposed method with a maximum average value of the absolute measured error below 0.04. More importantly, the proposed method is well-performed for the reconstruction of Stokes vectors at low light fields of various levels without requiring changes to the model, enhancing its practicality and simplicity.

Evaluation of a polarization-enhanced laparoscopy prototype for improved intra-operative visualization of peritoneal metastases

Despite careful staging, the accuracy for preoperative detection of small distant metastases remains poor, creating a clinical need for enhanced operative staging to detect occult peritoneal metastases. This study evaluates a polarization-enhanced laparoscopy (PEL) prototype and assesses its potential for label-free contrast enhancement of peritoneal metastases. This is a first-in-human feasibility study, including 10 adult patients who underwent standard staging laparoscopy (SSL) for gastrointestinal malignancy along with PEL. Image frames of all detectable peritoneal lesions underwent analysis. Using Monte Carlo simulations, contrast enhancement based on the color dependence of PEL (mPEL) was assessed. The prototype performed safely, yet with limitations in illumination, fogging of the distal window, and image co-registration. Sixty-five lesions (56 presumed benign and 9 presumed malignant) from 3 patients represented the study sample. While most lesions were visible under human examination of both SSL and PEL videos, more lesions were apparent using SSL. However, this was likely due to reduced illumination under PEL. When controlling for such effects through direct comparisons of integrated (WLL) vs differential (PEL) polarization laparoscopy images, we found that PEL imaging yielded an over twofold Weber contrast enhancement over WLL. Further, enhancements in the discrimination between malignant and benign lesions were achieved by exploiting the PEL color contrast to enhance sensitivity to tissue scattering, influenced primarily by collagen. In conclusion, PEL appears safe and easy to integrate into the operating room. When controlling for the degree of illumination, image analysis suggested a potential for mPEL to provide improved visualization of metastases.

Efficient acquisition of Mueller matrix via spatially modulated polarimetry at low light field

Mueller polarimetry performed in low light field with high speed and accuracy is important for the diagnosis of living biological tissues. However, efficient acquisition of the Mueller matrix at low light field is challenging owing to the interference of background-noise. In this study, a spatially modulated Mueller polarimeter (SMMP) induced by a zero-order vortex quarter wave retarder is first presented to acquire the Mueller matrix rapidly using only four camera shots rather than 16 shots, as in the state of the art technique. In addition, a momentum gradient ascent algorithm is proposed to accelerate the reconstruction of the Mueller matrix. Subsequently, a novel adaptive hard thresholding filter combined with the spatial distribution characteristics of photons at different low light levels, in addition to a low-pass fast-Fourier-transform filter, is utilized to remove redundant background noise from raw-low intensity distributions. The experimental results illustrate that the proposed method is more robust to noise perturbation, and its precision is almost an order of magnitude higher than that of the classical dual-rotating retarder Mueller polarimetry at low light field.

Mueller matrix polarization imaging and quantitative parameters analysis method

Mueller matrix polarization imaging is a new biomedical optical imaging method that can generate both polarization and isotropic intensity images of structures of the biological tissue sample surface. In this paper, a Mueller polarization imaging system in the reflection mode is described for obtaining the Mueller matrix of the specimens. Diattenuation, phase retardation, and depolarization of the specimens are derived by using the conventional Mueller matrix polarization decomposition method and a newly proposed direct method. The results show that the direct method is more convenient and faster than the conventional decomposition method. The polarization parameter combination method is then presented in which any two of the diattenuation, phase retardation, and depolarization parameters are combined, and three new quantitative parameters are defined in order to reveal more detailed anisotropic structures. The images of in vitro samples are presented to demonstrate the capability of the parameters introduced.

Compressive space-dimensional dual-coded hyperspectral polarimeter (CSDHP) and interactive design method

A compressive space-dimensional dual-coded hyperspectral polarimeter (CSDHP) and interactive design method are introduced. A digital micromirror device (DMD), a micro polarizer array detector (MPA), and a prism grating prism (PGP) are combined to achieve single-shot hyperspectral polarization imaging. The longitudinal chromatic aberration (LCA) and spectral smile of the system are both eliminated to guarantee the matching accuracy of DMD and MPA pixels. A 4D data cube with 100 channels and 3 Stocks parameters is reconstructed in the experiment. The feasibility and fidelity are verified from the image and spectral reconstruction evaluations. It is demonstrated that the target material can be distinguished by CSDHP.

Numerical study of a high-extinction-ratio micro-polarizer array with a metal grid substrate

In this paper, a complete investigation and simulation of the extinction ratio (ER) in a setup with a micro-polarizer array (MPA) is performed. A metal grid substrate structure, which can effectively reduce optical crosstalk and improve the ER, is proposed. The numerical calculation results show that the ER of our proposed structure is more than 10-fold higher than that of the traditional structure. Moreover, the effect of its structural parameters to the ER has been researched, and the results showed that when the substrate is thicker, the incident angle of light is less than 15°, the metal width is larger than 900 nm, and a complete grating with more than 20 periods on a single pixel can obtain a high ER.

Error model for linear DoFP imaging systems perturbed by spatially varying polarization states

Division of focal plane (DoFP) polarization sensors can perform linear polarimetric imaging in one shot. However, since they use several neighboring pixels to estimate the polarization state, fast spatial variations of the scene may lead to estimation errors. We investigate the influence of the spatial variations of the three polarimetric parameters of interest (intensity, degree of linear polarization, and angle of polarization) on these errors. Using theoretical derivations and imaging experiments, we demonstrate that the spatial variations of intensity are the main source of estimation errors, much more than variations in the polarization state. Building on this analysis, we show that compensating the intensity variations within a superpixel is sufficient to reach the estimation performance of state-of-the-art demosaicing methods.

Dual division of focal plane polarimeters-based collinear reflection Mueller matrix fast imaging microscope

SIGNIFICANCE

Reflection Mueller matrix imaging is suitable for characterizing the microstructure of bulk specimens and probing dynamic processes in living animals, there are always demands for speed and accuracy for such applications to avoid possible artifacts and reveal a sample's intrinsic properties.

AIM

To demonstrate a design of collinear reflection Mueller matrix fast imaging microscope based on dual division of focal plane (DoFP) polarimeters (DoFPs-CRMMM) which has high measurement speed and accuracy.

APPROACH

In DoFPs-CRMMM, to improve the measurement speed, we applied the dual DoFP polarimeters design on the collinear reflection system for the first time to achieve fast imaging in about 2 s. To improve the measurement accuracy, we improved the double-pass eigenvalue calibration method (dp-ECM) by background light correction, and explored the optimization of the set of reference samples.

RESULTS

DoFPs-CRMMM was applied to measure the standard polarization samples and monitor the tissue optical clearing process of an artificial layered bulk tissue. Results show that the system has satisfactory performance which can capture the variation of polarization properties during the dynamic process.

CONCLUSIONS

We present the establishment and demo application of DoFPs-CRMMM. The measurement speed can be further accelerated for potential applications in monitoring dynamic processes or living biomedical systems.

Correlative imaging of the spatio-angular dynamics of biological systems with multimodal instant polarization microscope

The spatial and angular organization of biological macromolecules is a key determinant, as well as informative readout, of their function. Correlative imaging of the dynamic spatio-angular architecture of cells and organelles is valuable, but remains challenging with current methods. Correlative imaging of spatio-angular dynamics requires fast polarization-, depth-, and wavelength-diverse measurement of intrinsic optical properties and fluorescent labels. We report a multimodal instant polarization microscope (miPolScope) that combines a broadband polarization-resolved detector, automation, and reconstruction algorithms to enable label-free imaging of phase, retardance, and orientation, multiplexed with fluorescence imaging of concentration, anisotropy, and orientation of molecules at diffraction-limited resolution and high speed. miPolScope enabled multimodal imaging of myofibril architecture and contractile activity of beating cardiomyocytes, cell and organelle architecture of live HEK293T and U2OS cells, and density and anisotropy of white and grey matter of mouse brain tissue across the visible spectrum. We anticipate these developments in joint quantitative imaging of density and anisotropy to enable new studies in tissue pathology, mechanobiology, and imaging-based screens.

Self-calibration for Mueller polarimeters based on DoFP polarization imagers

Mueller polarimeters (MPs) based on division of focal plane (DoFP) polarization imagers can achieve fast measurements and significantly improve the effectiveness of Mueller polarimetry. In this Letter, we demonstrate a unique property of the DoFP sensor-based MPs: they can be calibrated without any extra polarizing reference element. We describe a self-calibration method that only requires six image acquisitions; based on our analysis, the calibration accuracy is only limited by the noise.

Definition of an error map for DoFP polarimetric images and its application to retardance calibration

With the recent development of division of focal plane (DoFP) polarization sensors, it is possible to perform polarimetric analysis of a scene with a reduced number of acquisitions. One drawback of these sensors is that polarization estimation can be perturbed by the spatial variations of the scene. We thus propose a method to compute a map that indicates where polarization estimation can be trusted in the image. It is based on two criteria: the consistency between the intensity measurements inside a super-pixel and the detection of spatial intensity variations. We design both criteria so that a constant false alarm rate can be set. We demonstrate the benefit of this method to improve the precision of dynamic retardance calibration of DoFP-based full Stokes imaging systems.

Polarization enhanced laparoscope for improved visualization of tissue structural changes associated with peritoneal cancer metastasis

A polarization enhanced laparoscopy (PEL) imaging system was developed to examine the feasibility of utilizing PEL to augment conventional white light laparoscopy (WLL) in the visualization of peritoneal cancer metastases. The system includes a modified tip to illuminate tissue with linearly polarized light and elements in the detection path enabling recording of corresponding images linearly co- and cross-polarized relative to the incident light. WLL and PEL images from optical tissue phantoms with features of distinct scattering cross-section confirm the enhanced sensitivity of PEL to such characteristics. Additional comparisons based on images acquired from collagen gels with different levels of fiber alignment highlight another source of PEL contrast. Finally, PEL and WLL images of ex vivo human tissue illustrate the potential of PEL to improve visualization of cancerous tissue surrounded by healthy peritoneum. Given the simplicity of the approach and its potential for seamless integration with current clinical practice, our results provide motivation for clinical translation.

Development of an all-day portable polarization lidar system based on the division-of-focal-plane scheme for atmospheric polarization measurements

A portable polarization lidar system based on the division-of-focal-plane scheme has been proposed for all-day accurate retrieval of the atmospheric depolarization ratio. The polarization lidar system has been designed as a T-shaped architecture consisting of a closed transmitter and a detachable large focal receiver, which is capable of outdoor unmanned measurements. The lidar system features low cost, low maintenance and short blind range (∼100 m) by utilizing a 450 nm multimode laser diode as the light source and a polarization image sensor with four polarized channels as the detector. Validation measurements have been carried out on a near horizontal path in ten consecutive days. The linear volume depolarization ratio (LVDR) as well as its measurement uncertainty has been theoretically and experimentally evaluated without employing additional optical components and sophisticated online calibrations. The offset angle can also be accurately retrieved (i.e., -0.06°) from the four-directional polarized lidar profiles with a standard deviation of ±0.02° during the whole measurement period, which contributes negligible influence on the retrieval of the LVDR. It has been found out that the uncertainty of the LVDR was mainly originated from the random noise, which was below 0.004 at nighttime and may reach up to 0.008 during daytime owing to the increasing sunlight background. The performance of the polarization lidar system has been further examined through atmospheric vertical measurements. The low-cost low-maintenance portable polarization lidar system, capable of detecting four-directional polarized lidar signals simultaneously, opens up many possibilities for all-day field measurements of dust, cloud, urban aerosol, oriented particles, etc.

Fast Mueller matrix microscope based on dual DoFP polarimeters

In this Letter, we report a dual division of focal plane (DoFP) polarimeters-based full Mueller matrix microscope (DoFPs-MMM) for fast polarization imaging. Both acquisition speed and measurement accuracy are improved compared with those of a Mueller matrix microscope based on dual rotating retarders. Then, the system is applied to probe the polarization properties of a red blood cells smear. The experimental results show that a DoFPs-MMM has the potential to be a powerful tool for probing dynamic processes in living cells in future studies.

Integration time optimization and starting angle autocalibration of full Stokes imagers based on a rotating retarder

Full Stokes imaging can be performed with a continuously rotating retarder in front of a fixed polarizer and a standard camera (RRFP) or a division of a focal plane polarization camera (RRDOFP). We determine the optimal number and duration of intensity measurements through a cycle of the retarder for these two types of setups as a function of instrument and noise parameters. We show that this number mainly depends on the type of noise that corrupts the measurements. We also show that with these setups, the starting angle of the retarder need not be known precisely and can be autocalibrated, which facilitates synchronization of the rotating retarder with the camera. We investigate the precision and feasibility domain of this autocalibration and show the RRDOFP setup has more attractive properties compared with RRFP setup. These results are important to optimize and facilitate the operation of polarization imagers based on a rotating retarder.

Constructing a portable optical polarimetry probe for in-vivo skin cancer detection

SIGNIFICANCE

Management of skin cancer worldwide is often a challenge of scale, in that the number of potential cases presented outweighs the resources available to detect and treat skin cancer.

AIM

This project aims to develop a polarimetry probe to create an accessible skin cancer detection tool.

APPROACH

An optical probe was developed to perform bulk tissue Stokes polarimetry, a technique in which a laser of known polarization illuminates a target, and the altered polarization state of the backscattered light is measured. Typically, measuring a polarization state requires four sequential measurements with different orientations of polarization filters; however, this probe contains four spatially separated detectors to take four measurements in one shot. The probe was designed to perform at a lower cost and higher speed than conventional polarimetry methods. The probe uses photodiodes and linear and circular film polarizing filters as detectors, and a low-coherence laser diode as its illumination source. The probe design takes advantage of the statistical uniformity of the polarization speckle field formed at the detection area.

RESULTS

Tests of each probe component, and the complete system put together, were performed to evaluate error and confirm the probe's performance despite its low-cost components. This probe's potential is demonstrated in a pilot clinical study on 71 skin lesions. The degree of polarization was found to be a factor by which malignant melanoma could be separated from other types of skin lesions.

Review on Complete Mueller Matrix Optical Scanning Microscopy Imaging

Optical scanning microscopy techniques based on the polarization control of the light have the capability of providing non invasive label-free contrast. By comparing the polarization states of the excitation light with its transformation after interaction with the sample, the full optical properties can be summarized in a single 4×4 Mueller matrix. The main challenge of such a technique is to encode and decode the polarized light in an optimal way pixel-by-pixel and take into account the polarimetric artifacts from the optical devices composing the instrument in a rigorous calibration step. In this review, we describe the different approaches for implementing such a technique into an optical scanning microscope, that requires a high speed rate polarization control. Thus, we explore the recent advances in term of technology from the industrial to the medical applications.

Optical Technologies for the Improvement of Skin Cancer Diagnosis: A Review

The worldwide incidence of skin cancer has risen rapidly in the last decades, becoming one in three cancers nowadays. Currently, a person has a 4% chance of developing melanoma, the most aggressive form of skin cancer, which causes the greatest number of deaths. In the context of increasing incidence and mortality, skin cancer bears a heavy health and economic burden. Nevertheless, the 5-year survival rate for people with skin cancer significantly improves if the disease is detected and treated early. Accordingly, large research efforts have been devoted to achieve early detection and better understanding of the disease, with the aim of reversing the progressive trend of rising incidence and mortality, especially regarding melanoma. This paper reviews a variety of the optical modalities that have been used in the last years in order to improve non-invasive diagnosis of skin cancer, including confocal microscopy, multispectral imaging, three-dimensional topography, optical coherence tomography, polarimetry, self-mixing interferometry, and machine learning algorithms. The basics of each of these technologies together with the most relevant achievements obtained are described, as well as some of the obstacles still to be resolved and milestones to be met.

Linear diattenuation imaging of biological tissues with near infrared Mueller scanning microscopy

Among the multitude of optical polarization contrasts that can be observed in complex biological specimens, linear diattenuation (LD) imaging has received little attention. It is indeed challenging to image LD with basic polarizing microscopes because it is often relatively small in comparison with linear retardance (LR). In addition, interpretation of LD images is not straightforward when experiments are conducted in the visible range because LD can be produced by both dichroism and anisotropic scattering. Mueller polarimetry is a powerful implementation of polarization sensing able to differentiate and measure the anisotropies of specimens. In this article, near infrared transmission Mueller scanning microscopy is used to image LD in thin biological specimen sections made of various proteins with unprecedented resolution and sensitivity. The near infrared spectral range makes it possible to lower the contribution of dichroism to the total linear diattenuation in order to highlight anisotropic scattering. Pixel-by-pixel comparison of LD images with LR and multiphoton images demonstrates that LD is produced by under-resolved structures that are not revealed by other means, notably within the sarcomere of skeletal muscles. LD microscopy appears as a powerful tool to provide new insights into the macro-molecular organization of biological specimens at the sub-microscopic scale without labelling.

Snapshot spectral polarimetric light field imaging using a single detector

In this Letter, we investigate a snapshot spectral-polarimetric-volumetric imaging (SSPVI) system using a single detector. Through compressed acquisition and reconstruction, SSPVI can achieve spectral imaging (x,y,λ), polarization imaging (x,y,ψ,χ), and light field imaging (x,y,θ,φ) simultaneously. The newly discovered performance is showcased by attaining the spectral-polarimetric-volumetric video and different laboratory accuracy experiments. These never-seen-before capacities of the camera open new prospects for many applications, such as biological analysis, object recognition, and remote sensing.

When is retardance autocalibration of microgrid-based full Stokes imagers possible and useful?

Full Stokes polarimetric images can be obtained from two acquisitions with a microgrid polarization camera equipped with a retarder. When the retardance is imperfectly known, it can be calibrated from the measurements, but this requires three image acquisitions and may cause divergence of estimation variance at a low signal-to-noise ratio. We determine closed-form equations allowing one to decide in which experimental conditions autocalibration is possible and useful, and to quantify the performance gain obtained in practice. These results are validated by real-world experiments.

Learning-based denoising for polarimetric images

Based on measuring the polarimetric parameters which contain specific physical information, polarimetric imaging has been widely applied to various fields. However, in practice, the noise during image acquisition could lead to the output of noisy polarimetric images. In this paper, we propose, for the first time to our knowledge, a learning-based method for polarimetric image denoising. This method is based on the residual dense network and can significantly suppress the noise in polarimetric images. The experimental results show that the proposed method has an evident performance on the noise suppression and outperforms other existing methods. Especially for the images of the degree of polarization and the angle of polarization, which are quite sensitive to the noise, the proposed learning-based method can well reconstruct the details flooded in strong noise.

Theory of autocalibration feasibility and precision in full Stokes polarization imagers

We propose a general theory of simultaneous estimation of Stokes vector and instrumental autocalibration of polarization imagers. This theory is applicable to any polarization imager defined by its measurement matrix. We illustrate it on the example of retardance autocalibration in a large class of polarization imagers based on rotating retarders and polarimeters. We show that although all these architectures can yield optimal estimation precision of the Stokes vector if they are properly configured, they do not have the same autocalibration capacity and have to be specifically optimized for that purpose. These results are important to determine the best compromise between autocalibration capacity and polarimetric precision in practical applications.

Precision of retardance autocalibration in full-Stokes division-of-focal-plane imaging polarimeters

We investigate the validity domain and precision of retardance autocalibration in full-Stokes imaging polarimeters based on a linear division-of-focal-plane polarization camera. We demonstrate that the level of precision of autocalibration in these systems gets worse as the degree of linear polarization of input Stokes vector approaches zero. Autocalibration is impossible when the input is purely circular or totally unpolarized. In all other cases, reaching a given level of precision requires a higher signal-to-noise ratio as the input gets closer to circular or unpolarized.

Fundamental precision limits of full Stokes polarimeters based on DoFP polarization cameras for an arbitrary number of acquisitions

As an emerging technology, division-of-focal-plane (DoFP) polarization cameras have raised attention due to their integrated structure. In this paper, we address the fundamental precision limits of full Stokes polarimeters based on a linear DoFP polarization camera and a controllable retarder in the presence of additive and Poisson shot noise. We demonstrate that if the number of image acquisitions is greater than or equal to three, there exists retarder configurations that reach the theoretical lower bound on estimation variance. Examples of such configurations are one rotatable retarder with fixed retardance of 125.26° or two rotatable quarter-waveplates (QWPs) in pair. In contrast, the lower bound cannot be reached with a single QWP or a single variable retarder with fixed orientation. These results are important to get the most out of DoFP polarization imagers in real applications.

Structured illumination Mueller matrix imaging

We perform Mueller matrix imaging (MMI) of diffusely scattering phantoms under sinusoidal irradiance of varying spatial frequency. Quantitative polarimetric sensing via MMI completely characterizes a sample's polarimetric properties, while structured illumination (SI) allows for the control of photon path length. Intralipid phantoms were measured with varying absorption and with varying depth to demonstrate photon path length control for Mueller matrix elements. We observe unpolarized intensity, linear polarization, and circular polarization to depend upon spatial frequency differently. Finally, we measured an ex vivo chicken skin sample over a bright and dark substrate to further demonstrate the sensitivity of SI-MMI to depth.

Degree of optical polarization as a tool for detecting melanoma: proof of principle

Determining the optical polarization properties of a skin lesion is a proposed method to differentiate melanoma from other skin lesions. We developed an in vivo Stokes polarimetry probe that fires a laser of known polarization at the skin and measures the Stokes parameters of the backscattered light in one shot. From these measured Stokes parameters, we can calculate the degree of polarization (DOP). Through testing on rough skin phantoms, a correlation between backscattered DOP and skin roughness was identified for both linear and circular input polarization, the latter of which was found to be more useful. In a pilot clinical trial of 69 skin lesions in vivo, it was found that the mean DOP for melanoma (linear input on melanoma: 0.46  ±  0.09) was greater than that of other lesions (linear input on all other lesions: 0.28  ±  0.01). This separation is greater for circular polarized input light, and it is likely that circular polarized light's greater sensitivity to surface roughness contributes to this result. In addition, all skin lesions demonstrated a stronger depolarizing effect on circular polarized light than linear polarized light. We have identified DOP as a potentially useful measurement to identify melanoma among other types of skin lesions.

Distinguishing anisotropy orientations originated from scattering and birefringence of turbid media using Mueller matrix derived parameters

Anisotropic structures such as myofibrils, collagen, and elastic fibers are prevalent in tissues. The orientations of these anisotropic structures are important indicators in various biomedical studies. Here we analyze the ability of using Mueller matrix polar decomposition (MMPD) and Mueller matrix transformation (MMT) parameters to determine and distinguish the accurate orientations of different anisotropies originated from scattering and birefringence in backscattering measurements. The experimental results of complex phantoms and Monte Carlo simulations suggest that the MMT and MMPD parameters can be used to reveal the orientations of the cylindrical scatterers and birefringence in turbid media, respectively. Moreover, a preliminary application of these Mueller matrix-derived parameters on bovine tendon samples demonstrates the ability of using the parameters to distinguish anisotropic scattering and birefringence orientations of tissues. The anisotropy orientation information can be helpful for biomedical studies or diagnosis.

Full-Stokes temporal imaging

We developed a full-Stokes temporal imaging system which measures the Stokes vector of ultrafast signals as a function of time. The system is based on a time-lens array where each time-lens in the array projects the signal on a different state of polarization.

Assessment of tissue polarimetric properties using Stokes polarimetric imaging with circularly polarized illumination

Tissue-depolarization and linear-retardance are the main polarization characteristics of interest for bulk tissue characterization, and are normally interpreted from Mueller polarimetry. Stokes polarimetry can be conducted using simpler instrumentation and in a shorter time. Here, we use Stokes polarimetric imaging with circularly polarized illumination to assess the circular-depolarization and linear-retardance properties of tissue. Results obtained were compared with Mueller polarimetry in transmission and reflection geometry, respectively. It is found that circular-depolarization obtained from these 2 methods is very similar in both geometries, and that linear-retardance is highly quantitatively similar for transmission geometry and qualitatively similar for reflection geometry. The majority of tissue circular-depolarization and linear-retardance image information (represented by local image contrast features) obtained from Mueller polarimetry is well preserved from Stokes polarimetry in both geometries. These findings can be referred to for further understanding tissue Stokes polarimetric data, and for further application of Stokes polarimetry under the circumstances where short acquisition time or low optical system complexity is a priority, such as polarimetric endoscopy and microscopy.